1. Field of the Invention
The present invention relates to a voltage-controlled oscillator, and more particularly, to a voltage-controlled oscillator for improving a phase noise characteristic.
2. Description of the Related Art
Like most other electronic parts used for mobile communication devices, it is strongly required for voltage-controlled oscillators (VCOs) to be small and light in addition to low current consumption and low voltage operation. The VCO is a variable frequency oscillation circuit module for stably oscillating the transmitting frequency and the receiving local oscillation frequency of a mobile phone that is one of the mobile communication devices, by the application voltage of a frequency synthesizer. The VCO is used as one of the very important parts of a frequency synthesizer circuit in the mobile communication devices.
The frequency synthesizer is used as a circuit for automatically converting a frequency to an assigned phone channel for the convenience of use in the mobile communication device. The frequency synthesizer is typically constituted by a local oscillator.
A complementary metal oxide semiconductor (CMOS) type including an LC resonator is mainly used for the VCO for a high frequency. The CMOS circuit technology is widely and commercially used because of low static current consumption. Furthermore, since most semiconductor technologies converge into a CMOS technology, the CMOS circuit technology is advantageous not only for high reliability in a manufacturing process, but also for high integration.
There have been many studies to maximize the performance of the CMOS type VCO circuit including an LC resonator while maintaining the advantages of the CMOS circuit. Among them, since a phase noise characteristic is important to an oscillator circuit which operates in a radio frequency (RF) range, a study to improve the phase noise characteristic is also actively performed.
A generally well known phase noise model is introduced in a thesis entitled “A Simple Model of Feedback Oscillator Noise Spectrum” by Leeson published in Proceedings of the IEEE, vol. 54, pp. 329-330, 1966, which is represented in Equation 1.
                              L          ⁡                      (                          ω              m                        )                          ∝                              1                          V              o              2                                ·                      KT            C                    ·                                    (                                                ω                  o                                Q                            )                        2                    ·                      1                          ω              m              2                                                          [                  Equation          ⁢                                          ⁢          1                ]            
Here, V0 denotes an oscillator output voltage, KT denotes a thermal noise, C denotes an equivalent capacitance, ω0 denotes a resonant frequency, Q denotes Q factor, the performance of an oscillator, and ωm denotes a noise frequency.
This model clearly shows that the phase noise has an inclination of 1/f, 1/f2, and 1/f3 based on the structures of the resonance circuit and oscillator. However, the Lesson model has a drawback in that it cannot clearly calculate a final phase noise value of the oscillator with an equation. To overcome the drawback, circuit designers have experimentally obtained the final phase noise value by appropriately determining an introduced proportional constant F.